How TF do you have a hidden room that big in a house??? I can understand a basement or smaller room above ground, but if a room is too big, it gets obvious when you walk around and notice a huge discrepancy in the floor layout. "whats with this huge vacant square wall with no door leading inside?" Based on the average Japanese sized house, that room looks like it takes a quarter, a fifth at the very least, of whatever floor it's on... Imagine if your garage had no doors and was your "secret" room, that's how obvious it is.
You may fool the average guest (why would they be inspecting your floor layout), but if you live there... I call BS.